At present, thin film transistors (TFTs) in which amorphous silicon or low-temperature polysilicon is used for semiconductor layers are being widely used as switching devices or driving devices in display devices such as active matrix liquid crystal display devices or organic electroluminescent (EL) devices.
However, since a high temperature process is required for producing such TFTs, it is difficult to use flexible substrates having low heat resistance such as plastic substrates or film substrates.
When amorphous silicon TFTs are used as driving devices of organic EL devices, since the field-effect mobility is low (up to 1 cm2V−1 s−1), TFTs having a large size are required and it is difficult to reduce the size of pixels. Furthermore, there is also a problem in that driving of TFTs for a long period of time causes a change in the threshold voltage of the TFTs and current passing through organic EL devices decreases.
As for low-temperature polysilicon TFTs, since correction circuits are required to overcome nonuniformity caused by an excimer laser used when crystallizing silicon, circuits become complicated. In addition, there is, for example, the following problem: since the size of a display is restricted by the radiation size of excimer laser, it is difficult to achieve a large display size.
On the other hand, in recent years, a technique of using an amorphous oxide semiconductor composed of In, Ga, Zn, and O for a channel layer of TFTs has been studied.
Oxide semiconductor TFTs are very promising as switching devices or driving devices instead of amorphous silicon TFTs or low-temperature polysilicon TFTs for display apparatuses including flexible substrates or organic EL devices.
However, oxide semiconductors containing ZnO have high sensitivity to oxygen, moisture, and the like contained in the atmosphere depending on the composition of the semiconductors and there are cases where electrical characteristics of the semiconductors change. Accordingly, to achieve stable use as thin film transistors, it is necessary to protect semiconductor layers from the atmosphere by using protective layers constituted by insulating layers.
When such protective layers are formed by a plasma chemical vapor deposition method (CVD method), a sputtering method, or the like, characteristics of TFTs are degraded by, for example, damage caused by plasma to channel layers composed of oxide semiconductors or diffusion of hydrogen from the protective layers. To avoid such degradation of characteristics, a method of suppressing degradation of characteristics of a TFT by making a channel layer composed of an oxide semiconductor have a bilayer structure and making the carrier concentration of the upper layer lower than the carrier concentration of the lower layer has been disclosed (Patent Literature 1). At this time, the carrier concentration is controlled by performing doping with a carrier acceptor such as Cu. In addition, a technique of enhancing device characteristics by making a channel layer serving as an active layer contain hydrogen in a predetermined concentration has been disclosed (Patent Literature 2).
However, when amorphous oxide semiconductor TFTs are used as driving devices of organic EL devices, the stability of the threshold voltage when driving is performed for a long period of time is not necessarily sufficient and there are cases where a circuit for correcting the threshold voltage is required. Accordingly, further enhancement of the stability against electric stress is desired.
To improve the stability of the threshold voltage against electric stress, use of oxide semiconductor channel layers having high mobility is effective. However, on the other hand, since oxide semiconductor channel layers having high mobility also have low electrical resistance, it is difficult to turn off the drain current of TFTs and variation in the threshold voltage also increases.
In summary, there is a tradeoff relationship between the stability of the threshold voltage against electric stress and variation in the threshold voltage. When one characteristic is enhanced, the other characteristic is degraded. Thus, it is difficult to achieve both of the characteristics in good states.
For example, to suppress variation in the threshold voltage while electrical resistance is low, a method of making the film thickness of a channel layer small is effective. However, when the film thickness of a channel layer is made smaller than a certain film thickness, there are cases where the stability of the threshold voltage against electric stress is degraded by the above-described damage upon the formation of the protective layer.
As one method for overcoming the tradeoff relationship, a method of making a channel layer have a bilayer structure as described in Patent Literature 1 is effective. Specifically, a thin low-resistance layer is formed as a channel layer on the gate insulating layer side and a high-resistance layer is formed as a channel layer on the protective layer side. As a result of this method, since the effective film thickness is small, variation in the threshold voltage can be suppressed to a certain degree while the stability of the threshold voltage against electric stress is maintained. However, the control of the resistance is performed in Patent Literature 1 by performing doping with a carrier acceptor such as Cu. Use of a target containing a carrier acceptor such as Cu, introduction of a gas containing a carrier acceptor, or ion implantation after film formation causes an increase in the production costs.
An object achieved by the present invention is to provide, at low cost and without using a technique of performing doping with a carrier acceptor, an oxide semiconductor TFT in which change in the threshold voltage under electric stress and variation in TFT characteristics are small.